Tetris was one of those games that everybody seemed to have for Nintendo or Gameboy, yet nobody knew who bought it for them or why they had multiple copies. Just like games that come standard with your computer, Tetris is something that you can find yourself getting lost into for anywhere from a few minutes to a few hours. Tris for the iPhone is a decent Tetris clone.
Tris is pretty decent, but it Tetris doesn't really translate well to the iPhone. First off, you're simply moving the pieces around with your finger and tapping them to change their shape. If you've got big fingers, then expect to cover up pieces that you're looking to land on.
Also, considering there isn't any form of grid guiding you as you place down the blocks, chances are you'll end up with them in a completely different place then you had intended. This isn't a huge issue at lower levels and speeds, but as the game picks up you're kind of boned.
For a free game, Tris is enjoyable. But all of the little hiccups are exactly why this game isn't something that we'd recommend you try. If you're looking for a puzzle game there are plenty others out there.